In the prior art, it is known to record and playback signals representing high density information by focusing laser light to a small spot on the surface of an optical disc. As an example of an ordinary optical disc record and playback apparatus, reference is made to U.S. Pat. No. 4,426,693. A prior art optical disc is enclosed in U.S. Pat. No. 4,624,914. These prior art patents disclose write-once optical discs, and a record and playback apparatus for use with such write-once type optical discs. A write-once type disc, as the name implies, is a disc which allows information to be written only once on a track thereof, although the information may thereafter be repeatedly read.
In recent years, there have also been proposed erasable type optical discs which permit information to be repeatedly written and erased on the same tracks. References describing erasable discs include "Million Cycle Overwritable Phase Change Optical Disk Media", SPIE Optical Data Storage Technical Digest Series, Volume 1, p. 14 TU A4-1 (1989) and "In-Se Based Phase Change Reversible Optical Recording Film", SPIE Proc. 695, p. 105 (1986).
Since write-once discs and erasable discs record and reproduce signals on a thin film recording layer by use of focused laser light, their recording and playback principles are very similar. However, prior art record and playback systems designed for use with write-once discs cannot be used with erasable discs.